The process of this invention is an improvement over the disclosures of U.S. Pat. Nos. 4,247,678 and 4,496,707. In particular, this invention relates to an improved process for preparing polyurethane products of the type described in U.S. Pat. No. 4,496,707, the improvement comprising the use of a mixture of an aromatic diamine and an aliphatic diamine ketimine curing agent.
It is known to use aromatic diamines as curing agents for polyurethane cements and that the use of aliphatic diamines for this purpose is undesirable because exceedingly fast rates of cure usually occur when the aliphatic diamines are used. Although aromatic diamines are preferred as curing agents for polyurethane cements, they give a slow rate of cure. Ketimines of aromatic diamines have been described as components of certain flexible sealants and corrosion inhibitive primers used to protect the exterior skin and fastener patterns of high performance aircraft at low temperatures. These sealants and primers which are composed of an epoxy resin, pigments including corrosion inhibitor, polyurethane elastomer, ketimine curing agent, and solvents for sprayability are disclosed in U.S. Pat. No. 4,101,497.
Bicyclic aromatic diamines, such as 4,4'-methylene-bis(2-chloraniline) (MOCA), 4,4'-methylenedianiline (MDA), and 4,4'-diaminodiphenylsulfone (SDA) can be used as curing agents for polyurethanes. It would be highly desirable to minimize the use of the bicyclic aromatic diamines and to use in their place to the extent possible, the aliphatic diamines such as metaxylene diamine (MXDA), isophorone diamine (IPDA), 1,3-bis(aminomethyl)cyclohexane (1,3 BAC) and others which have less apparent carcinogenic potential than the aromatic diamines.
Unfortunately, aliphatic diamines cause much faster rates of cure in polyurethane cements than the aromatic diamines do. This undesirable property has seriously limited the use of aliphatic diamines as curing agents in polyurethane cements.